Just to bump the thread once more, here is another interesting comment I came up with when searching "morphine potentiate" on google:
"Any acidifying agents here is not recommended Acidifying agents such as citric acid found in grapefruit juice. In fact, the opposite is recommended. That is-alkylnizing agents such as those found in Tums/Rolaids along with CIMETADINE/Tagament not Pepcid AC Bro. Pepcid is not an alkylnizing agent neither is Cimetadine, but the way Cimetadine suppresses acid production jives really well with the metabolizing of opiates/opiods and thus, potentiates the action thereof and increases(doubles or more) the half-life of such metabolites.
Benedryl I know for a fact DOES NOT potentiate opiates/opiods. You know why it even ever came to be discussed that the youngins at Erowid weren't/aren't aware of?? It came onto the scene as a way to take tar heroin and using Bendryl and a coffee grinder you could produce a snortable form of tar chiva. So somehow it got crossed over into pharmaceutical opiates/opiods and through urban mythologicals and legends has found it's place where it is today in this present day and age."